Hydrogeology and Irrigated Agriculture

A review of drainage problems and groundwater processes on the west side of the San Joaquin Valley (SJV), California, provides useful perspectives on hydrogeology and irrigated agriculture. Fundamentally, drainage problems arise from increases in water table elevation which in turn arise from excess groundwater recharge relative to the discharge capacity of the underlying aquifer. This imbalance in the groundwater budget demands that the problem be addressed, in part, through regional hydrogeologic analysis, The water table can be lowered by correcting the imbalance through one or a combination of the following: (1) reduce amount of applied water to reduce groundwater recharge, (2) install drains to increase groundwater discharge, and (3) increase groundwater pumping. To effect more than mere site-specific results, these measures should be performed at a regional scale. Otherwise, the regional groundwater budget will not be altered sufficiently. In the SJV, increased flow to drains is undesirable because disposal of this poor quality water is problematic. Modeling studies have shown that discharge to drains and amount of land subject to evaporation from the water table can be reduced dramatically by simultaneously reducing applied water (through land retirement and improved irrigation efficiency) and increasing groundwater pumpage by prudent amounts. Contrary to conventional thinking, but consistent with knowledge of generic basin hydrogeologic processes, pumping from existing wells tapping deep, confined aquifers can be very effective in such a scheme - perhaps more effective than shallow groundwater pumping. Although such a groundwater management approach can effectively lower the water table and mitigate drainage problems, the potential exists that downward movement of shallow, poor-quality groundwater will be accelerated. Ultimate impacts on deeper groundwater quality may not occur for 1 Os to 100s of years, and the potential magnitude of such impacts remains unclear. Consequently, advanced modeling techniques are needed for forecasting potential long-term effects of such practices on basin groundwater quality. In many basins, the key issue may be how much faster the water quality degradation will occur rather than whether the water quality degradation will occur as a result of increased groundwater pumping.